The subject matter disclosed herein relates to the use of energy-discriminating photon-counting detectors, including silicon-based photon-counting detectors, such as in material decomposition contexts.
Non-invasive imaging technologies allow images of the internal structures or features of a subject (patient, manufactured good, baggage, package, or passenger) to be obtained non-invasively. In particular, such non-invasive imaging technologies rely on various physical principles, such as the differential transmission of X-rays through the target volume or the reflection of acoustic waves, to acquire data and to construct images or otherwise represent the internal features of the subject.
For example, in X-ray-based imaging technologies, X-ray radiation spans a subject of interest, such as a human patient, and a portion of the radiation impacts a detector where the intensity data is collected. In digital X-ray systems, a detector produces signals representative of the amount or intensity of radiation impacting discrete pixel regions of a detector surface. The signals may then be processed to generate an image that may be displayed for review.
In one such X-ray based technique, known as computed tomography (CT), a scanner may project fan-shaped or cone-shaped X-ray beams from an X-ray source at numerous view angle positions about an object being imaged, such as a patient. The X-ray beams are attenuated as they traverse the object and are detected by a set of detector elements which produce signals representing the intensity of the incident X-ray intensity on the detector. The signals are processed to produce data representing the line integrals of the linear attenuation coefficients of the object along the X-ray paths. These signals are typically called “projection data” or just “projections”. By using reconstruction techniques, such as filtered backprojection, images may be generated that represent a volume or a volumetric rendering of a region of interest of the patient or imaged object. In a medical context, pathologies or other structures of interest may then be located or identified from the reconstructed images or rendered volume.
Conventionally, radiation detectors used in these types of imaging techniques operate in an energy-integrating (i.e., readout of the total integrated energy deposited during an acquisition interval) mode or a photon-counting (each individual X-ray photon is detected and its energy characterized) mode. Energy integration is the conventional mode for X-ray detectors in most clinical applications. However, energy-integrating readout approaches operate poorly in low-flux imaging applications, where electronic noise associated with the detector, including the readout operation, may overwhelm the available signal. As evident to those skilled in the art, photon-counting detectors offer other benefits relative to energy-integrating detectors, such as improved resolution, the ability to improve contrast-to-noise ratio by optimally weighting detected photons, the ability to better delineate materials in the X-ray beam, and so on.
In some applications, photon counts are of more interest than the total integrated energy information associated with energy-integrating approaches. Conventional scintillator-based photon-counting detectors for positron emission tomography (PET) utilize silicon photomultipliers (SiPMs) that are expensive and not practical for high count rate applications such as CT. Further, some photon-counting approaches may be limited in the type of information they produce, such as yielding only raw photon count data without information pertaining to the energy of the detected photons.
In contrast, certain techniques, such as dual-energy (e.g., high- and low-energy imaging) and/or material-decomposition imaging, benefit not only from photon counts in a general sense, but from obtaining spectral information for a given exposure interval. That is, such techniques utilize photon counts that are broken down into respective energy bins, and thus discriminate between photon events at different energies, thereby determining and counting the number of photons observed at different photon energy ranges. To address this need, certain energy-discriminating, photon-counting X-ray detector technologies may be employed. In certain instances, such approaches employ a detection medium that directly converts incident X-rays to measurable signal (i.e., electron-hole pairs generated using direct conversion materials), as opposed to techniques employing a scintillator-based intermediary conversion and subsequent detection of the generated optical photons.
Examples of such direct conversion materials include cadmium zinc telluride (CZT) and cadmium telluride (CdTe). However, these materials are not capable of higher incident count rates that may be of interest in practice. Alternatively, silicon strips may be employed as part of a direct-conversion, energy-discriminating, photon-counting detector. Such silicon strip based detectors may be capable of higher incident photon count rates than CZT or CdTe detectors. However, the primary attenuation mechanism with silicon strip based detectors is Compton scattering, which can substantially decrease dose efficiency and spectral fidelity in the associated energy response function of the detector. In particular, such a Compton scatter event typically results in an X-ray photon transferring a portion of its energy to a particle with which it interacts in passing (i.e., the deflecting or scatting particle), thereby changing (i.e., decreasing) the energy of the X-ray photon as well as potentially changing its trajectory.